U.S. patent application number 13/075364 was filed with the patent office on 2012-10-04 for method for determining a location of a mobile unit using a system reference time maintained by the mobile unit.
Invention is credited to Subramanian Vasudevan, Jialin Zou.
Application Number | 20120252463 13/075364 |
Document ID | / |
Family ID | 46045081 |
Filed Date | 2012-10-04 |
United States Patent
Application |
20120252463 |
Kind Code |
A1 |
Zou; Jialin ; et
al. |
October 4, 2012 |
METHOD FOR DETERMINING A LOCATION OF A MOBILE UNIT USING A SYSTEM
REFERENCE TIME MAINTAINED BY THE MOBILE UNIT
Abstract
The present invention provides a method of determining the
location of a mobile node. Embodiments of the method may include
comparing, at the mobile node, a local timing reference maintained
by the mobile node to arrival times of signals transmitted by two
or more base stations to determine two or more distances between
the mobile node and the base stations. Embodiments of the method
may also include determining, at the mobile node, a location of the
mobile node using the distances and locations of the base
stations.
Inventors: |
Zou; Jialin; (Randolph,
NJ) ; Vasudevan; Subramanian; (Morristown,
NJ) |
Family ID: |
46045081 |
Appl. No.: |
13/075364 |
Filed: |
March 30, 2011 |
Current U.S.
Class: |
455/441 ;
455/456.6 |
Current CPC
Class: |
G01S 5/14 20130101; G01S
11/08 20130101; G01S 5/021 20130101 |
Class at
Publication: |
455/441 ;
455/456.6 |
International
Class: |
H04W 64/00 20090101
H04W064/00; H04W 36/32 20090101 H04W036/32 |
Claims
1. A method, comprising: comparing, at a mobile node, a global
system timing reference locally maintained by the mobile node to
arrival times of signals transmitted by at least two base stations
to determine at least two distances between the mobile node and
said at least two base stations; and determining, at the mobile
node, a location of the mobile node using said at least two
distances and locations of said at least two base stations.
2. The method of claim 1, wherein determining the location of the
mobile node comprises using trilateration to determine the location
of the mobile node using said at least two distances and the
locations of said at least two base stations.
3. The method of claim 1, comprising receiving information
indicating the locations of said at least two base stations from
one of said at least two base stations, wherein said information is
broadcast by one of said at least two base stations.
4. The method of claim 1, comprising synchronizing the
locally-maintained global timing reference to a global timing
reference used to coordinate signals transmitted by said at least
two base stations, wherein the locally-maintained global timing
reference is synchronized to the global timing reference at
intervals defined by a drift speed of the local timing reference
and a distance measurement error.
5. The method of claim 4, wherein synchronizing the
locally-maintained global timing reference to the global timing
reference comprises synchronizing the locally-maintained global
timing reference using a timing offset determined by a serving base
station using signals received from the mobile node.
6. The method of claim 4, wherein synchronizing the
locally-maintained global timing reference to the global timing
reference comprises synchronizing the locally-maintained global
timing reference to a timing reference signal provided by a global
positioning system.
7. The method of claim 1, comprising determining at least one speed
of the mobile node using at least two locations of the mobile node
that are determined at times separated by a selected time interval,
wherein said at least two locations of the mobile node are each
determined using at least two distances between the mobile node and
at least two base stations and at least two locations of said at
least two base stations.
8. The method of claim 7, comprising determining a scaling factor
for a handoff hysteresis used by the mobile node, wherein the
scaling factor is determined as a substantially continuous function
of said at least one speed.
9. The method of claim 1, comprising transmitting, from the mobile
node, information indicating the location of the mobile node and an
identifier of a femtocell, wherein said information is transmitted
in response to determining, at the mobile node, that a signal
strength from the femtocell has increased above a predetermined
threshold.
10. The method of claim 9, wherein said information is transmitted
to at least one of the femtocell or one of said at least two base
stations to indicate the location of the femtocell.
11. The method of claim 9, comprising receiving, at the mobile
node, information indicating locations of at least one femtocell,
wherein said information is broadcast by one of said at least two
base stations.
12. A method, comprising: providing, from a base station in
response to a request from a mobile node, a timing offset defined
to synchronize a global timing reference maintained locally by the
mobile node to a global timing reference used to transmit signals
from the base station, wherein the mobile node is configured to
compare the locally-maintained global timing reference to arrival
times of signals transmitted by the base station and at least one
other base station to determine at least two distances between the
mobile node and the base stations and thereby determine a location
of the mobile node using said at least two distances and locations
of the base stations.
13. The method of claim 12, comprising providing, from the base
station, information indicating the locations of the base station
and said at least one other base station.
14. The method of claim 12, comprising receiving, from the mobile
node, information indicating the location of the mobile node and an
identifier of at least one femtocell, wherein said information is
transmitted in response to determining, at the mobile node, that a
signal strength from said at least one femtocell has increased
above a predetermined threshold.
15. The method of claim 14, comprising storing information
associating the location of the mobile node with at least one
location of said at least one femtocell.
16. The method of claim 15, comprising broadcasting information
indicating said at least one location of said at least one
femtocell.
17. The method of claim 16, wherein broadcasting said information
comprises broadcasting information indicating a latitude range and
a longitude range that encompasses at least one of a micro-cell, a
pico-cell, or said at least one femtocell.
18. A method, comprising: storing, at a femtocell, information
indicating the location of the femtocell, wherein said information
is determined by a mobile node in response to the mobile node
determining that a signal strength transmitted by the femtocell is
above a predetermined threshold, and wherein the location of the
femtocell is determined by a location determined by the mobile node
using locations of at least two base stations and at least two
distances between the mobile node and said at least two base
stations, and wherein said at least two distances are determined by
comparing, at the mobile node, a global system timing reference
maintained locally by the mobile node to arrival times of signals
transmitted by said at least two base stations.
19. The method of claim 18, further comprising receiving, at the
femtocell and from a serving base station, said information
indicating the location of the femtocell.
20. The method of claim 19, wherein storing said information
indicating the location of the femtocell comprises information
indicating the location of the mobile node and information
indicating a one-way delay between the serving base station and the
femtocell.
Description
BACKGROUND
[0001] 1. Field of the Invention
[0002] This invention relates generally to communication systems,
and, more particularly, to wireless communication systems.
[0003] 2. Description of the Related Art
[0004] A conventional communication system uses one or more access
nodes to provide network connectivity to one or more mobile nodes.
The access nodes may be referred to as access points, access
networks, base stations, base station routers, cells, macrocells,
microcells, femtocells, pico-cells, and the like. For example, in a
cellular communication system that operates according to Universal
Mobile Telecommunication Services (UMTS) standards, one or more
nodes may be used to provide wireless network connectivity to
mobile nodes. The mobile nodes may include cellular telephones,
personal data assistants, smart phones, text messaging devices,
Global Positioning Systems, navigation systems, network interface
cards, notebook computers, desktop computers, and the like.
Numerous types and generations of wireless communication systems
have been developed and deployed to provide network connectivity to
mobile nodes. Exemplary wireless communication systems include
systems that provide wireless connectivity to micro cells (e.g.,
systems that provide wireless connectivity according to the IEEE
802.11, IEEE 802.15, or Wi-Fi standards) and systems that provide
wireless connectivity to macro cells (e.g., systems that operate
according to the Third Generation Partnership Project
standards--3GPP, 3GPP2--and/or systems operate according to the
IEEE 802.16 and IEEE 802.20 standards). Multiple generations of
these systems have been deployed including Second Generation (2G),
Third
[0005] Generation (3G), and Forth Generation (4G) standards.
[0006] The coverage provided by different service providers in a
heterogeneous communication system may intersect and/or overlap.
For example, a wireless access node for a wireless local area
network may provide network connectivity to mobile nodes in a
microcell or pico-cell associated with a coffee shop that is within
the macrocell coverage area associated with a base station of a
cellular communication system. For another example, cellular
telephone coverage from multiple service providers may overlap and
mobile nodes may therefore be able to access the wireless
communication system using different generations of radio access
technologies, e.g., when one service provider implements a 3G
system and another service provider implements a 4G system. For yet
another example, a single service provider may provide coverage
using overlaying radio access technologies, e.g., when the service
provider has deployed a 3G system and is in the process of
incrementally upgrading to a 4G system.
[0007] Service providers and/or third parties support numerous
applications and services that use the location and/or the speed of
the mobile node. Mobile nodes that operate according to existing
standards estimate their speed by counting the number of handovers
between different cells. This approach consumes a relatively low
amount of overhead and processing resources, but it provides a very
inaccurate estimate of the speed of the mobile node that limits the
functionality of the mobile node.
[0008] FIG. 1 conceptually illustrates a conventional wireless
communication system 100. In the illustrated embodiment, mobile
nodes 105, 110 are moving to the wireless communication system 100
at substantially the same speed along parallel paths 115, 120. Both
of the mobile nodes 105, 110 are configured to estimate their speed
by counting the number of handovers between the cells 125. During a
time interval used to calculate the speed, the mobile node 105
crosses 5 different cell boundaries as it travels along the path
115. In contrast, the mobile node 110 only crosses two cell
boundaries during the same time interval as it travels along the
path 120. Consequently, the mobile node 105 estimates its speed to
be more than twice the speed of the mobile node 110, even though
the mobile nodes 105, 110 are traveling at substantially the same
speed. Estimating the speed in the manner depicted in the
illustrated embodiment also assumes that the cells 125 have the
substantially the same shape and size, which is assumed to remain
constant in time. This assumption rarely (if ever) holds true in an
actual wireless communication system. Moreover, the sizes of the
macrocells and microcells in heterogeneous networks are expected to
differ by orders of magnitude, which would further degrade the
accuracy of the conventional technique for estimating the speed of
the mobile nodes 105, 110.
[0009] The low accuracy of the speed estimation limits the
functionality of the mobile node. For example, the 3GPP Long Term
Evolution (LTE) standards only define high, medium, low speeds and
a scaling factor is defined for each level or category respectively
at least in part because of the low accuracy of the technique for
determining the speed of the mobile node. In systems that operate
according to LTE, a speed or velocity of the mobile node can be
used to determine scaling factors for scaling the hysteresis
parameters used when the mobile nodes hand off between cells.
Mobile nodes in the different speed categories multiply their
hysteresis parameters by different scaling factors. For example,
for high speed mobile nodes, performing faster handovers to avoid
handover failures is a primary concern and ping-ponging of the
mobile node between cells is a secondary concern and so handover
sensitivity should be increased for high speed mobile nodes. The
scaling factor therefore makes the hysteresis parameter smaller for
higher speed mobile nodes. In contrast, ping-ponging is a primary
concern for slower moving mobile nodes (and faster handovers may be
a secondary concern) so the handover sensitivity may be decreased
for slower moving mobile nodes. The scaling factor for the slower
categories makes the hysteresis parameter larger for lower speed
mobile nodes. However, the coarse division of mobile nodes into
only three categories limits the ability of the system to adapt to
variations in the speed of the mobile node.
[0010] Improving the accuracy of the estimated speed of mobile
nodes typically incurs high costs in the form of increased overhead
and/or increased power consumption within the mobile node. For
example, mobile nodes may be equipped with global positioning
system (GPS) functionality that can be used to determine the
location of a mobile node to high accuracy. However, each location
determination requires powering up the GPS in the mobile node,
acquiring signals from several GPS satellites, and then performing
the location calculation based on the received satellite signals.
The GPS functionality consumes a significant amount of battery
power to perform each location determination and overall power
consumption may become prohibitive when frequent location
estimations are performed. For example, the battery capacity in a
mobile node may not be sufficient to support the frequent location
estimation sampling that would be needed to estimate the velocity
of the mobile node using the GPS functionality.
[0011] Location estimation may also be performed using measurements
of radio signals transmitted between mobile nodes and base stations
in the wireless communication system.
[0012] For example, the network can estimate the location of a
mobile node using trilateration and/or triangulation based on
measurements of an observed time difference of arrival (OTDOA) or
an uplink time difference of arrival (UTDOA) for signals
transmitted from the mobile node to base stations in the network.
However, the air interface overhead required to support frequent
location estimation may be prohibitive. For example, frequent
location estimation will increase the measurement reporting
overhead needed to transport the accurate raw data from mobile
nodes to the network to support network-side trilateration
calculations. Frequent location determinations will also increase
the network backhaul traffic required to deliver measurement
results from base stations to a central network entity (such as a
radio network controller) that performs the triangulation
calculations on the network side. For example, the network requires
measurement information from at least three base stations to
estimate the location of each mobile node. Consequently,
significant complexity and processing power are required at the
network to estimate the locations of a large population of mobile
nodes. Moreover, many applications require that the mobile node
know its location and/or speed. Consequently, the network may have
to deliver the location and/or speed information to each mobile
node over the air interface, thereby additionally increasing
overhead.
[0013] Network-side location determination techniques are further
complicated by requiring communication pathways over the air
interface. Active mobile nodes may have a connection to the network
that allows the mobile node to provide measurement reports to the
network.
[0014] The active connection also allows the network to report the
location and/or speed information back to the mobile node. However,
idle mobile nodes do not have an active connection and so they
cannot deliver the information that the network needs to estimate
the location and/or speed of the idle mobile node. Furthermore,
even if the network acquires this information, the network would
have to locate and wake the mobile node to return the location
and/or speed information to the idle mobile node.
SUMMARY OF EMBODIMENTS OF THE INVENTION
[0015] The disclosed subject matter is directed to addressing the
effects of one or more of the problems set forth above. The
following presents a simplified summary of the disclosed subject
matter in order to provide a basic understanding of some aspects of
the disclosed subject matter. This summary is not an exhaustive
overview of the disclosed subject matter.
[0016] It is not intended to identify key or critical elements of
the disclosed subject matter or to delineate the scope of the
disclosed subject matter. Its sole purpose is to present some
concepts in a simplified form as a prelude to the more detailed
description that is discussed later.
[0017] In one embodiment, a method is provided for determining the
location of a mobile node. Embodiments of the method may include
comparing, at the mobile node, a local timing reference maintained
by the mobile node to arrival times of signals transmitted by two
or more base stations to determine two or more distances between
the mobile node and the base stations. Embodiments of the method
may also include determining, at the mobile node, a location of the
mobile node using the distances and locations of the base
stations.
[0018] In another embodiment, a method is provided for supporting
mobile node location determinations. Embodiments of the method may
include providing, from a base station in response to a request
from a mobile node, a timing offset defined to synchronize a local
timing reference maintained by the mobile node to a global timing
reference used to transmit signals from the base station. The
mobile node is configured to compare the local timing reference to
arrival times of signals transmitted by the base station and one or
more other base station to determine distances between the mobile
node and the base stations and thereby determine a location of the
mobile node using the distances and locations of the base
stations.
[0019] In yet another embodiment, a method is provided for
determining the location of a femtocell. Embodiments of the method
may include storing, at a femtocell, information indicating the
location of the femtocell. The information is determined by a
mobile node in response to the mobile node determining that a
signal strength transmitted by the femtocell is above a
predetermined threshold. The location of the femtocell is
determined by a location determined by the mobile node using
locations of base stations and distances between the mobile node
and the base stations. The distances are determined by comparing,
at the mobile node, a local timing reference maintained by the
mobile node to arrival times of signals transmitted by the base
stations.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The disclosed subject matter may be understood by reference
to the following description taken in conjunction with the
accompanying drawings, in which like reference numerals identify
like elements, and in which:
[0021] FIG. 1 conceptually illustrates a conventional wireless
communication system;
[0022] FIG. 2 conceptually illustrates a first exemplary embodiment
of a wireless communication system;
[0023] FIG. 3 conceptually illustrates a second exemplary
embodiment of a wireless communication system;
[0024] FIG. 4 conceptually illustrates one exemplary embodiment of
a timing diagram;
[0025] FIG. 5 conceptually illustrates one exemplary embodiment of
a method for determining a location of a mobile node;
[0026] FIG. 6 conceptually illustrates a third exemplary embodiment
of a wireless communication system;
[0027] FIG. 7 conceptually illustrates a fourth exemplary
embodiment of a wireless communication system; and
[0028] FIG. 8 conceptually illustrates a fifth exemplary embodiment
of a wireless communication system.
[0029] While the disclosed subject matter is susceptible to various
modifications and alternative forms, specific embodiments thereof
have been shown by way of example in the drawings and are herein
described in detail. It should be understood, however, that the
description herein of specific embodiments is not intended to limit
the disclosed subject matter to the particular forms disclosed, but
on the contrary, the intention is to cover all modifications,
equivalents, and alternatives falling within the scope of the
appended claims.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
[0030] Illustrative embodiments are described below. In the
interest of clarity, not all features of an actual implementation
are described in this specification. It will of course be
appreciated that in the development of any such actual embodiment,
numerous implementation-specific decisions should be made to
achieve the developers' specific goals, such as compliance with
system-related and business-related constraints, which will vary
from one implementation to another. Moreover, it will be
appreciated that such a development effort might be complex and
time-consuming, but would nevertheless be a routine undertaking for
those of ordinary skill in the art having the benefit of this
disclosure.
[0031] The disclosed subject matter will now be described with
reference to the attached figures. Various structures, systems and
devices are schematically depicted in the drawings for purposes of
explanation only and so as to not obscure the present invention
with details that are well known to those skilled in the art.
Nevertheless, the attached drawings are included to describe and
explain illustrative examples of the disclosed subject matter. The
words and phrases used herein should be understood and interpreted
to have a meaning consistent with the understanding of those words
and phrases by those skilled in the relevant art. No special
definition of a term or phrase, i.e., a definition that is
different from the ordinary and customary meaning as understood by
those skilled in the art, is intended to be implied by consistent
usage of the term or phrase herein. To the extent that a term or
phrase is intended to have a special meaning, i.e., a meaning other
than that understood by skilled artisans, such a special definition
will be expressly set forth in the specification in a definitional
manner that directly and unequivocally provides the special
definition for the term or phrase.
[0032] Generally, the present application describes embodiments of
techniques that allow a mobile node to determine its location using
an internal system timing reference signal/clock that is
synchronized to a global system timing reference with substantially
zero timing offset. The mobile node can measure the delay between
the internal system timing reference and the arrival times of
signals transmitted by neighboring base stations. Since the
internal system timing reference is synchronized to the global
system timing reference that is used to coordinate transmissions
from the base stations, the measured delays are approximately equal
to the distance between the mobile node and the transmitting base
station divided by the signal propagation speed. The locations of
the neighboring base stations can be provided to the mobile node,
which is then able to calculate its location using the delays and
locations of at least two neighboring base stations, e.g., using
trilateration and/or triangulation. Embodiments of this technique
can provide accurate location determinations at a significantly
reduced overhead and power consumption compared to conventional
location determination techniques. For example, the mobile node can
determine its location without powering up a GPS system or
exchanging messages with the network, except for relatively
infrequent synchronization messages and/or messages that may be
used to convey the locations of the neighboring base stations to
the mobile node.
[0033] FIG. 2 conceptually illustrates a first exemplary embodiment
of a wireless communication system 200. In the illustrated
embodiment, a mobile node 205 is configured to communicate with one
or more base stations 210 by exchanging radiofrequency signals over
an air interface. The mobile node 205 and the base stations 210 may
communicate according to agreed upon standards and/or protocols,
such as the standards and/or protocols defined by 3GPP, IEEE, or
other standards bodies. Techniques for communicating over air
interfaces are well known and in the interest of clarity only those
aspects of wireless communication that are relevant to the claimed
subject matter will be discussed herein. The base stations 210
maintain a global timing reference that can be used to coordinate
signal transmission and/or reception throughout the wireless
communication system 200. For example, the base stations 210 may be
synchronized to a GPS global timing reference or some other global
timing reference.
[0034] In the illustrated embodiment, the mobile node 205 includes
an internal clock that is synchronized to the global timing
reference at selected intervals. For example, the internal clock
may be synchronized to the global timing reference so that the
internal clock represents a locally-maintained global system timing
reference signal with substantially zero offset. As used herein,
the term "substantially zero offset" is intended to indicate that
the locally-maintained global system timing reference signal is
synchronized to the global timing reference within a selected
tolerance. For example, the locally-maintained global system timing
reference signal may drift over time but may remain synchronized to
within a tolerance determined by a target or maximum value of a
distance measurement error that may result from a difference
between the locally-maintained global timing reference signal and
the global timing reference, as discussed herein.
[0035] FIG. 3 conceptually illustrates a second exemplary
embodiment of a wireless communication system 300. In the
illustrated embodiment, the wireless communication system 300
includes a mobile node 305 and a base station 310 that may be the
serving base station for the mobile mode 305. The mobile node 305
includes GPS functionality 315 and an internal clock 320. The base
station 310 also includes GPS functionality 325 and an internal
clock 330. The entities in the wireless communication system 300
are synchronized to a global timing reference provided by GPS
satellites 335 that broadcast GPS signals 340. Although a single
GPS satellite 335 is shown in FIG. 3, persons of ordinary skill in
the art should appreciate that a GPS system includes numerous
satellites and entities in the wireless communication system 300
typically acquire signals from two or more GPS satellites 335 to
determine their locations and/or timing references.
[0036] The base station 310 may remain substantially continuously
synchronized with the global timing reference provided by the GPS
system so that the internal clock 330 is locked to the global
timing reference. However, synchronizing the internal clock 320 to
the global timing reference provided by the GPS system may require
powering up the GPS functionality 315, acquiring signals 340
provided by several GPS satellites 335, and performing the clock
synchronization before powering down the GPS functionality 315. If
the GPS is turned on frequently, this procedure can consume a
significant amount of battery power and consequently reduce the
operating time of the mobile node 305. At least in part to conserve
battery power, the mobile node 305 may synchronize the internal
clock 320 to the global timing reference at selected time intervals
that may be many orders of magnitude longer than the clock period
determined by the clock frequency. The mobile node 305 can
synchronize to a global timing reference by system time calibration
from the attached GPS receiver (functionality) 315. To obtain the
GPS time that is used as the global system time, the GPS
functionality 315 only needs to acquire the GPS signals from two
satellites. On the other hand, signals from at least 4 satellites
have to be acquired by the GPS functionality 315 to perform the
location estimation. Consequently, when the mobile node 305 only
uses the GPS functionality 315 to track timing (and not to
determine its location), the GPS functionality 315 in the mobile
node 305 consumes less power than would be consumed if the mobile
node 305 used the GPS functionality 315 to perform a complete
location estimation. Therefore, relative to mobile nodes that use
their internal GPS functionality to calculate their location,
embodiments of the mobile node 305 may save a significant amount of
power by restricting use of the GPS functionality 315 to infrequent
calibration of the internal system time of the mobile node 305.
[0037] In the illustrated embodiment, the internal clock 320 is
synchronized to the global timing reference at time intervals that
are determined by the drift speed of the internal clock 320 and a
target distance measurement error. For example, the clock frequency
drift may be approximately 100 ppb (parts per billion) per Celsius
degree of temperature change. Persons of ordinary skill in the art
having benefit of the present disclosure should appreciate that the
drift speed of a clock depends on the clock design, the precise
structure of the actual clock, the temperature range, and other
variables. The term "approximately" is intended to encompass these
expected clock-to-clock variations in the drift speed. A typical
rate for temperature change experienced by a mobile node 305 may be
less than or on the order of 1 degree Celsius per hour, which would
result in approximately 100 ppb frequency drift per hour due to the
temperature change. A frequency drift of this magnitude translates
to a timing drift of approximately 10 -7 second per hour, which
equates (using the signal propagation speed) to a distance
measurement error of approximately 30 m per hour or 0.5 m per
minute. Synchronizing the internal clock 320 every 10 minutes may
therefore be able to maintain the maximum distance measurement
error at or below approximately 5 m. This may be sufficient for the
mobile node 305 since a reasonable estimate of the walking speed of
a pedestrian is approximately 3 km/h=50 m/min.
[0038] FIG. 4 conceptually illustrates one exemplary embodiment of
a timing diagram 400. In the illustrated embodiment, reference
timing signal boundaries are indicated by the arrows 405. In the
interest of clarity only one arrow 405 is specifically indicated by
a numeral in FIG. 4. The reference timing signal boundaries 405 can
be defined in a variety of ways. For example, the reference timing
signal boundaries 405 can be defined in terms of a frame structure,
a subframe structure, a time-division multiplexing (TDM) slot, and
the like. The reference timing signal boundaries 405 can be
detected in received signals using a variety of well known
techniques. For example, a receiver can detect a reference timing
signal boundaries 405 using a RAKE receiver that monitors a down
link pilot signals modulated with the orthogonal codes. The RAKE
receiver correlates the received signals to known orthogonal codes.
Peaks in the correlation function indicate arrival times of the
orthogonal codes. However, persons of ordinary skill in the art
should appreciate that other techniques can be used to detect the
reference timing signal boundaries 405.
[0039] The timing diagram 400 illustrates the global system timing
reference 410, the local system timing 415 tracked by the mobile
node, the global system timing reference 420 maintained at the
mobile node, and pilot signal timing boundaries 425 received at the
mobile node from a neighboring base station. The global system
timing reference 410 may correspond to a GPS timing reference or
some other external timing reference that is used by base stations
to coordinate transmission and/or reception of signals. The local
system timing 415 is offset from the global system timing reference
410 by a timing offset 430 caused the one-way delay for signals
transmitted from a serving base station to the mobile node. In the
illustrated embodiment, the mobile node can recover and/or maintain
the global system timing reference 420 by shifting or correcting
the local system timing 415 using the timing offset 430, as
indicated by the arrow 435. For example, the mobile node may
receive signals indicating the one-way delay measured by the
serving base station using signals received from the mobile node.
For another example, the mobile node may shift or correct the local
system timing 415 using global system timing provided by a GPS
system.
[0040] The mobile node can determine the distance between the
mobile node and the base station by comparing the
locally-maintained global system timing reference 420 to the timing
boundaries 425 of signals such as the pilot signal transmitted from
the neighboring base station. In the illustrated embodiment, the
comparison reveals that the timing boundaries 425 are delayed
relative to the locally-maintained global system timing reference
420 by a delay 440. The distance between the mobile node and the
neighboring base station is equal to the delay 440 multiplied by
the propagation speed of the signal transmitted by the neighboring
base station. The mobile node can therefore determine the distance
directly from the comparison of the timing reference 420 and the
timing boundaries 425 without using any additional signaling
between the mobile node and the neighboring base station. Since the
base stations in the communication system are synchronized to the
global system timing reference 410, the same approach can be used
to determine distances to any neighboring base station.
[0041] Referring back to FIG. 2, the mobile node 205 can determine
the distances 215 to the base stations 210 using embodiments of the
techniques described herein. For example, the mobile node 205 can
determine the one-way delays (OWD11, OWD12, OWD13) for signals
transmitted from the base stations 210 and received at the mobile
node 205. The distances can then be calculated using the
radiofrequency propagation speed:
DIST11=RF propagation speed.times.OWD11
DIST12=RF propagation speed.times.OWD12
DIST13=RF propagation speed .times.OWD13
The locations (a.sub.i,b.sub.i) of the base stations 210 are also
provided to the mobile node 205. For example, a serving base
station 210 may provide a list of the base stations 210 that
neighbor the mobile node 205 and their coordinates to the mobile
node 205 by broadcasting the coordinates and identities of the
neighbor base stations. Alternatively, the location information can
be provided to the mobile node 205 using dedicated signaling if
mobile node 205 has an active connection. The locations and the
identities of the neighbor base stations 210 are not likely to
change rapidly and so this overhead signaling may be provided
relatively infrequently, e.g., at intervals on the scale of
minutes, hours, or even days depending on how widely, rapidly,
and/or frequently the mobile node 205 moves. The locations and the
distances can then be used to determine the location of the mobile
node 205 using trilateration equations:
(x1-a1) 2+(y1-b1) 2=DIST11 2
(x1-a2) 2+(y1-b2) 2=DIST12 2
(x1-a3) 2+(y1-b3) 2=DIST13 2
[0042] The number of base station distances and locations needed to
determine the location of the mobile node 205 can vary depending on
the particular circumstances. A minimum of two distances/locations
may be needed to determine the location of the mobile node 205.
However, since the measured distance only defines a circle about
the location of the base station 210 and the intersection of two
circles is two points, a third distance/location may be used to
break the degeneracy between the two points defined by the
intersection of the two circles. In one embodiment, the mobile node
205 can select the base stations 210 that are used to determine the
location of the mobile node 205. For example, the mobile node 205
may select two or three or more base stations 210 from a larger set
of neighbor base stations based on the received signal strengths,
e.g., the mobile node 205 may select the three base stations 210
that have the highest received signal strengths.
[0043] FIG. 5 conceptually illustrates one exemplary embodiment of
a method 500 for determining a location of a mobile node. In the
illustrated embodiment, the mobile node or user equipment (UE) is
in communication with one or more base stations or eNodeBs (eNB).
The serving base station initially provides (at 505) locations of
neighboring base stations to the mobile node. The location
information can be provided (at 505) by broadcasting or unicasting
the location information. The mobile node is maintaining a local
system time reference and so the mobile node can determine (at 510)
its location using the received base station location information
and delays between the local system time reference and the timing
of signals received from the serving base station and any other
neighboring base stations. The mobile node can optionally report
(at 515) this information to the base station, e.g., in response to
a request from the network.
[0044] In the illustrated embodiment, the mobile node may request
(at 520) reference timing information to correct or calibrate the
locally-maintained global system timing reference. In response to
the request, the base station can measure (at 525) a timing offset
using signals received from the mobile node. The timing offset
corresponds to the one-way delay (OWD_serving) from the serving
base station to the mobile node. The base station can then transmit
(at 530) information indicating the timing offset to the mobile
node so that the mobile node can correct or calibrate (at 535) the
locally-maintained global system timing reference. For example, the
mobile node may be tracking and be synchronized with the received
signal from the serving base station so that the mobile node can
obtain the macro system timing by shifting or correcting (at 535)
the local reference in lock by the value of OWD_serving received
from the base station.
[0045] Alternatively, if the system time is synchronized to GPS
timing and the mobile node includes GPS functionality, the mobile
node's local system time could be obtained and calibrated directly
from the GPS system. This approach would only require relatively
infrequent use of the mobile mode's GPS receiver and other GPS
functionality only for local system time calibration. In contrast,
relatively frequent location estimation and/or speed estimation may
be performed using the locally-maintained global system timing
reference, which significantly reduces the power consumption by the
GPS functionality while also providing accurate position
determinations. In one embodiment, the OWD may initially be
received from the serving base station to allow the mobile node to
develop the association between the macro system timing and GPS
timing reference. The location estimation error is normally
accumulated over time and is biased towards one direction due to
the system bias on timing offset measurement and the local
reference clock drifting.
[0046] Embodiments of the low-cost, low power consumption location
determination technique described herein may enable a mobile node
to make more frequent location determinations. This can facilitate
transport functionality that would be impractical or impossible if
mobile nodes that implement location determination technique that
have lower accuracy and/or higher power consumption. For example, a
mobile node can use frequent location determinations to make
significantly more accurate speed estimations. The current speed
can be estimated by making multiple location estimations within
short periods of time. In one embodiment, accurate location
estimations may preferentially be performed right after the
calibration. For speed estimation, if the location samples are
taken between short periods of time, the system bias and
accumulated clock drifting error can be canceled. The impact of the
clock drift on the speed estimation is less than the location
estimation.
[0047] Small cells such as femtocells are typically private so that
the owner's mobile node(s) could memorize the locations of a few
its own femto cells. However, when the small cell is a public small
cell, the mobile node(s) that may attempt to access the public
cells are not expected to be able to store the locations of all of
the potentially available small cells. Moreover, the base stations
don't like to broadcast a long list of their locations. In some
embodiments, frequent location determinations can therefore be used
to map locations of public small cells (such as microcells,
picocells, femtocells, and the like) that overlay other cells. This
information can then be used to generate maps of the public small
cells that overlay a macro-cellular deployment. The public small
cell maps can be transmitted or broadcast to mobile nodes so that
the mobile nodes may be able to decide when they should search for
the public small cell signals and when they should conserve power
by turning off their femtocell reception functionality.
[0048] FIG. 6 conceptually illustrates a third exemplary embodiment
of a wireless communication system 600. In the illustrated
embodiment, locations (a.sub.i,b.sub.i) of base stations 610 are
provided to a mobile node 605. At a first time T.sub.1, the mobile
node 605 can determine distances 615 to the base stations 610 using
embodiments of the techniques described herein. For example, the
mobile node 605 can determine the one-way delays (OWD11, OWD12,
OWD13) for signals transmitted from the base stations 610 and
received at the mobile node 605 and the distances can be calculated
using the radiofrequency propagation speed. The locations and the
distances can then be used to determine the location (x.sub.1,
y.sub.1) of the mobile node 605 using trilateration equations. At a
second time T.sub.2 subsequent to the first time, the mobile node
605 can determine new distances 620 to the base station 610 using
embodiments of the techniques described herein. The locations and
the distances can then be used to determine the subsequent location
(x.sub.2, y.sub.2) of the mobile node 605 using trilateration
equations. The mobile node 605 can use this information to estimate
its current speed as:
s = ( x 2 - x 1 ) 2 + ( y 2 - y 1 ) 2 T 2 - T 1 ##EQU00001##
In one embodiment, the mobile node 605 can also calculate its
velocity (e.g., its speed and direction, which may be represented
as a vector, {right arrow over (s)}, that has a magnitude:
s=|{right arrow over (s)}|) using well known techniques. In
alternative embodiments, multiple location estimation samples can
be gathered and statistically combined (e.g., averaged) to improve
the accuracy of the location and/or speed estimations, as well as
providing information indicating the likely errors and/or standard
deviation of the estimations.
[0049] In one embodiment, estimates of the speed performed
according to embodiments of the techniques described herein may be
used to define scaling factors for the handoff hysteresis used by
the mobile node 605. For example, the scaling factor may be defined
as a substantially continuous function of the speed:
K_sc=F(Speed_uc) such as a linear function or some other continuous
function. Alternatively, the scaling factor may be used to
categorize the speed of the mobile node but the granularity of the
categories may be increased (relative to the conventional high,
medium, and low speed categories) to reflect the improved accuracy
of the speed estimation.
[0050] FIG. 7 conceptually illustrates a fourth exemplary
embodiment of a wireless communication system 700. In the
illustrated embodiment, the wireless communication system 700 is
part of a heterogeneous network that includes a macro-cellular base
station 705 and one or more femtocells 710 that provide wireless
connectivity in microcells 715 that overlay the macro-cellular
deployment. In one embodiment, the base station 705 and the
femtocell 710 may be connected by a wired and/or wireless interface
720. A mobile node 725 is able to communicate with the base station
705 over an air interface 730 and to communicate with the femtocell
710 over an air interface 735. Persons of ordinary skill in the art
should appreciate that the terms microcell, femtocell, picocell,
and the like typically refer to the size of the coverage area
provided by the associated access point or base station. The
functionality of the access point, base station, or base station
router that provides wireless connectivity within these areas may
be similar.
[0051] In a heterogeneous network (HetNet) such as shown in FIG. 7,
the macro-cellular coverage may be overlapped with the femtocell
coverage, e.g., in residential areas. Most of the femtocells and/or
some public picocells are typically deployed in an indoor
environment and consequently it may be difficult for the network
and/or the indoor femtocells to determine the location of the
indoor femtocells. For example, small cells (such as femtocells
and/or picocells) that are deployed indoors may not be able to
acquire a sufficient number of GPS satellite signals to provide
accurate location determination, at least in part because the
strength of the GPS signals indoor is typically 30 dB lower than
the measured outdoor strength of the GPS signals at the same
location. Furthermore, due to cost constraints many femtocells may
not even have a GPS receiver installed. However, many applications
require that the network know the location of the femtocell, e.g.,
to support important applications such as the emergency service
E911. Femtocell location information may also be used to support
system performance improvement or optimization. For example,
location information can be used to perform a location based search
for power saving possibilities or location based power control for
interference mitigation.
[0052] Embodiments of the location determination technique
described herein can be used to support a mobile-assisted technique
for determining the location of microcells such as femtocells
and/or picocells. In one embodiment, locations (m.sub.1, n.sub.1)
of femtocells 710 can be determined as follows: [0053] 1. A
femtocell calibration mode is enabled and the mobile node 725
monitors the power/signal strength for the femtocell 710. [0054] 2.
When the mobile node 725 determines that the signal strength for
the femtocell 710 is over a threshold, indicating that the location
and the one way delay to the macrocell 705 of the mobile node 725
is approximately equal to the location and the one way delay of the
femtocell 710, the mobile node 725 may determine and report its
location, e.g., the location may be reported as part of a power
measurement report that is transmitted to the network. The serving
macrocell of the network may also be notified or informed of the
one way delay from the base station 705 to the mobile node 725.
[0055] 3. In one embodiment, the mobile node 725 reports the
location information to the femtocell 710. The femtocell 710 may
then forward the information to the network, e.g., over the
interface 720 to the base station 705. [0056] 4. In alternative
embodiments, the mobile node 725 reports the location information
and the associated femtocell identifier to the overlaid macro cell
705. The macro cell 705 may then forward the location information
and the macro-to-femto one way delay to the network and/or the
femtocell 710. [0057] 5. In either case, the femtocell 710 becomes
aware of its estimated location and can subsequently broadcast this
information for use by other mobile nodes. [0058] 6. In one
embodiment, the location information could help the associated
mobile node 725 build a "fingerprint" of the overlaid microcells.
The mobile node 725 may then refrain from searching for
micro-cellular coverage until it determines that is proximate to
the fingerprint of overlaid microcells.
[0059] FIG. 8 conceptually illustrates a fifth exemplary embodiment
of a wireless communication system 800. In the illustrated
embodiment, the wireless communication system 800 is part of a
heterogeneous network that includes a macro-cellular base station
805 that provides wireless connectivity to a macrocell 810. The
network also includes a cluster of campus-CSG cells/picocells that
provide wireless connectivity in microcells 815 that overlay the
macro-cellular deployment. When the cluster includes a large number
of microcells 815 such as public pico cells and campus CSG cells, a
mobile node may not be able to build a radio fingerprint for all
the microcells 815. Mobile nodes can locate the microcells 815
using a global search, but the search would have to be performed
substantially continuously, even in regions that do not include any
micro-cells. In one embodiment, the base station 805 may broadcast
a signal or message to indicate whether there any micro-cells are
overlaid with the macro-cell 810. Mobile nodes may then only search
for micro-cells when they receive a positive indication that
micro-cells are present in the macro-cell 801, which may save
battery power in the mobile node.
[0060] To further save mobile power, alternative embodiments of the
base station 801 may broadcast information indicating the locations
of the pico-cells 815. Mobile nodes may then search for micro-cells
when they receive an indication that they are proximate one or more
micro-cells 815. The mobile node may determine its proximity to the
cells 815 by comparing the location information to its internally
determined location, as discussed herein. However if there are too
many pico-cells 815, broadcasting all of their individual locations
may be costly in terms of air interface overhead. The base station
805 may therefore broadcast information indicating the range of the
coverage area(s) of the micro-cells 815. For example, the base
station 805 may broadcast information indicating a longitude range
820 and a latitude range 825 of the coverage area(s). In this
embodiment, location-aware mobile nodes may start to search for the
micro-cells 815 when they determine that they are proximate to
and/or within the coverage area defined by the longitude range 820
and latitude range 825.
[0061] In one exemplary embodiment, the base station 805 may learn
and broadcast picocell location information as follows:
[0062] 1. Base stations 805 in each macrocell 810 may broadcast
information indicating whether picocells 815 are deployed
overlapping the macrocell 810, e.g. the base station 805 may
broadcast `1` to indicate that picocells are overlaid with the cell
and the base station 805 may broadcast `0` to indicate that no
picocells are overlaid with the macrocell 810. If a mobile node
sees the indicator is set to `1`, the mobile node may start to
search for picocells 815 in the macrocell 810. If a mobile node
sees the indicator is set to `0`, the mobile node does not initiate
or perform a search. If the indicator is not present in any
broadcast messages, the mobile node can optionally search with a
longer DRX (or searching) cycle.
[0063] 2. Alternatively, the base station 805 in the macro cell 810
that is overlaid with picocells 815 may broadcast location
information of the picocells 815 to support power saving in the
mobile node.
[0064] 3. If there are a large number of picocells 815 within a
particular coverage area, the base station 805 in the overlaid
macrocell 810 may broadcast information indicating the range or
boundaries that encompass the combined coverage area of the
picocells 815.
[0065] 4. When the mobile node moves into the macrocell 810 that is
broadcasting picocell location information, the mobile node may
begin to monitor its location more frequently to determine whether
it is proximate or within the coverage area of one or more of the
picocells 815.
[0066] 5. When the mobile node determines that it is proximate or
within the coverage area of one or more of the picocells 815, the
mobile node may begin a search and acquisition process to identify
the picocells 115 and potentially handoff to one of the picocells
815.
[0067] 6. For mobile nodes that don't have the capability to detect
their location, the presence of the picocell location information
may serve as an indication of existence of the picocells 815 in the
macro cell 810.
[0068] Portions of the disclosed subject matter and corresponding
detailed description are presented in terms of software, or
algorithms and symbolic representations of operations on data bits
within a computer memory. These descriptions and representations
are the ones by which those of ordinary skill in the art
effectively convey the substance of their work to others of
ordinary skill in the art. An algorithm, as the term is used here,
and as it is used generally, is conceived to be a self-consistent
sequence of steps leading to a desired result. The steps are those
requiring physical manipulations of physical quantities. Usually,
though not necessarily, these quantities take the form of optical,
electrical, or magnetic signals capable of being stored,
transferred, combined, compared, and otherwise manipulated. It has
proven convenient at times, principally for reasons of common
usage, to refer to these signals as bits, values, elements,
symbols, characters, terms, numbers, or the like.
[0069] It should be borne in mind, however, that all of these and
similar terms are to be associated with the appropriate physical
quantities and are merely convenient labels applied to these
quantities. Unless specifically stated otherwise, or as is apparent
from the discussion, terms such as "processing" or "computing" or
"calculating" or "determining" or "displaying" or the like, refer
to the action and processes of a computer system, or similar
electronic computing device, that manipulates and transforms data
represented as physical, electronic quantities within the computer
system's registers and memories into other data similarly
represented as physical quantities within the computer system
memories or registers or other such information storage,
transmission or display devices.
[0070] Note also that the software implemented aspects of the
disclosed subject matter are typically encoded on some form of
program storage medium or implemented over some type of
transmission medium. The program storage medium may be magnetic
(e.g., a floppy disk or a hard drive) or optical (e.g., a compact
disk read only memory, or "CD ROM"), and may be read only or random
access. Similarly, the transmission medium may be twisted wire
pairs, coaxial cable, optical fiber, or some other suitable
transmission medium known to the art. The disclosed subject matter
is not limited by these aspects of any given implementation.
[0071] The particular embodiments disclosed above are illustrative
only, as the disclosed subject matter may be modified and practiced
in different but equivalent manners apparent to those skilled in
the art having the benefit of the teachings herein. Furthermore, no
limitations are intended to the details of construction or design
herein shown, other than as described in the claims below. It is
therefore evident that the particular embodiments disclosed above
may be altered or modified and all such variations are considered
within the scope of the disclosed subject matter. Accordingly, the
protection sought herein is as set forth in the claims below.
* * * * *